As usual the inspired writer concludes his narrative of the reign of Nadab by directing his readers to another source of information for that reign. In this case it is the Book of the Chronicles of the Kings of Israel.
This is a source that is no longer available to us; we do not know these chronicles.
31 Now the rest of the acts of Nadab and all that he did, are they not written in the Book of the Chronicles of the Kings of Israel?